Prevent DEFAULTOPTIONS mutation and validate max-width/height inputs

[shekhei]

Context

src/index.js uses Object.assign(DEFAULTOPTIONS, loaderUtils.getOptions(this)), which mutates the shared DEFAULTOPTIONS object across loader invocations. It also parses width/height with parseInt() without validation, which can yield NaN and still calls gm.resize().

Why this matters

Shared mutation can leak configuration between builds/runs, and invalid options can lead to confusing failures or unexpected output sizes.

Proposal

• Avoid mutating defaults by cloning options.
• Validate numeric width/height and provide clear errors or fallbacks.
• Use an explicit radix in parseInt or Number() with Number.isFinite checks.

Tasks

• Change option merge to Object.assign({}, DEFAULTOPTIONS, loaderOptions) (or spread).
• Validate max-width / max-height and handle invalid values (error or skip resize).
• Add tests to cover invalid or missing values and ensure defaults do not leak between runs.